The present invention relates to devices and methods for removing the residual torsion in optical-fiber ribbons of an optical fiber cable of the type used in the telecommunications industry.
Optical fiber cables of the type commonly used in the telecommunications industry comprise a high number of optical fibers organized in ribbons (usually made up of four fibers each), where each ribbon typically groups the fibers associated with two specific users. The optical fiber ribbons are properly housed inside grooves (usually five superimposed ribbons per groove) located on the peripheral portion of a central core of the cable, and are externally sheathed with one or more layers of insulating and protective material.
The grooves (usually five per cable) are angularly equidistant from one another, and they extend along the cable with a substantially helical progress. This configuration permits a relative displacement of the fibers with respect to each other, and with respect to central core of the cable, if the cable is subject to mechanical and/or thermal stresses. Such a configuration is advantageous in that the optical fibers are extremely delicate and are adversely affected by such stresses.
Optical fiber telecommunication networks are organized in a ring structure. The connection to the network is usually made after the line cable is laid. These connections are achieved through junctions which correspond with specific service areas provided along the ring line, and are usually sealed inside junction boxes.
For the purpose of allowing the connection of the various users to the communication network, the line cable must have, inside the junction boxes, some intermediate portions of fixed length (typically about five meters) in which the optical fiber cables are free from the external sheathing and the central core. This operation, commonly known as xe2x80x9ccable peelingxe2x80x9d, must be carried out without interrupting the physical continuity of the fiber ribbons of the communication network, so as to allow the continuing transmission of the signal or signals along the entire ring line. Thus, in the portions of xe2x80x9cpeeledxe2x80x9d cable located inside the junction boxes, the cable presents a plurality of intermediate portions of ribbon which extend uninterrupted from the grooves of the cable upstream from the junction box to the grooves of the cable downstream from the junction box. These portions are regularly collected in special housing units.
The intermediate portions of ribbon possess a residual torsion which arises from the helical progress of the grooves in which the ribbons upstream and downstream of the junction box are housed, and in which the same intermediate portions of ribbon were housed prior to the peeling of the cable. This residual torsion blocks the correct positioning on the housing units. Thus, it is necessary for the ribbons to have fixed-length portions which are free from residual torsion.
The prior art provides for the use of a comb-like tool provided with a plurality of longitudinal slots. Typically, the number of slots corresponds to the number of grooves on the optical-fiber cable. The comb is inserted between the ribbons inside the junction box, and is moved from one end of the junction box to the other. During the movement of the comb, the residual torsions are confined to the portions of ribbon downstream of the comb. Thus, the portions of ribbon downstream from the comb become increasingly piled up and twisted, even as the portions of ribbon upstream of the comb are freed from the residual torsion. At the end of the operation, there will be a skein of twisted ribbons downstream from the comb which is confined and enclosed in a space located at an end of the junction box. Due to the inherent fragility of the optical fibers within the ribbons (which, as noted above, do not endure mechanical stresses very well), the heaping of the ribbons in the skein can cause serious damage to the fibers, thereby adversely effecting the efficiency and quality of the signal transmission along the line.
There is thus a need in the art for a simple and effective method of removing the residual torsion in optical fiber ribbons which minimizes damage to the fibers and which therefore minimizes attenuation of the signal transmitted along the line. These and other needs are met by the present invention, as hereinafter described.
In one aspect, the present invention relates to a method and device for removing the residual torsion in the optical fiber ribbons of an optical fiber cable of the type commonly used in the telecommunications industry. Cables of this type typically comprise a central core, a plurality of substantially helical grooves located in the central core, a plurality of optical fiber ribbons located inside the grooves, and an external protective sheathing. The cable also typically comprises at least one intermediate section of fixed length, which includes portions of uncut ribbons that have been freed from the central core and from the external protective sheath, These portions of ribbon possess a residual torsion. In accordance with the present invention, a spool is provided which is adapted to be wound up with these portions of ribbons. In this way, the residual torsions of the optical-fiber ribbons are eliminated in an easy, practical and orderly manner without damaging the fibers or causing attenuation of the signal along the line.
During the installation of the network, the operator winds the ribbons onto the spool in a manner consistent with the helical pitch of the ribbons. The ribbons are neither cut nor interrupted. By use of the spool, the stress which the fibers are subjected to is minimized and is substantially lower than the stresses attendant to the prior art combing method. The use of the spool also eliminates any skein or entanglement of fibers inside the junction box, thus keeping the environment within the box cleaner and tidier and facilitating any inspection or maintenance operation thereon.
Preferably, the spool comprises a bottom plane, a guiding means for guiding the ribbon onto the spool, a ribbon housing means, and ribbon inlet and outlet openings to and from the guiding means and the ribbon housing means. Thus, the spool is extremely simple from the constructive point of view, and it can be readily produced from plastic materials by known molding processes.
The guiding and ribbon housing means preferably comprise first guiding walls which protrude perpendicularly from the bottom plane. The first guiding walls include a substantially circular central wall and have a fixed bending radius, and are also provided with a substantially annular peripheral wall which is concentric to the central wall. Thus, the ribbons are housed on a circular surface, and the housing is facilitated by the helical progress of the ribbons themselves. The bending radius of the central wall is preferably greater than some fixed minimum value. This protects the optical fibers within the ribbons from possible bending during installation and maintenance.
The ribbon inlet and outlet openings preferably comprise second guiding walls which protrude perpendicularly from the bottom plane. These walls include a substantially rectilinear base wall and upper walls that are substantially parallel to the base wall, and are radiused to the peripheral wall of the guiding means and the ribbon housing means. This design facilitates the introduction and extraction of the ribbons from the spool. Preferably, the spool is also devoid of any edges or acute angles which could damage the fibers.
The device of the present invention also preferably comprises holding means for holding the ribbons in position in the guiding means and the ribbon housing means. Such holding means advantageously comprises retaining tabs which protrude perpendicularly from the central wall and are substantially parallel to the bottom plane. The tabs are preferably interspaced from one another by about 90xc2x0. As a result, the positioning of the ribbons on the spool is extremely easy and safe. In the preferred embodiment, the positioning is further facilitated by the presence of notches on both the base and the peripheral wall which correspond with the tabs. The device of the present invention also preferably comprises means for clamping the spool to a support plane so as to ensure a safe positioning of the spool inside the junction box.
In another aspect, the present invention relates to a method for removing the residual torsion in optical fiber ribbons of an optical fiber cable of the type used in the telecommunications industry. The cable comprises a central core, a plurality of substantially helical grooves located in the central core, a plurality of optical fiber ribbons located inside the grooves, and an external protective sheathing. In accordance with the method, the central core and the protective sheathing is removed from a segment of the cable without cutting the ribbons, thereby freeing portions of the ribbons from the central core and from the protective sheathing. These portions of ribbon possess a residual torsion. Then, at least a portion of the ribbons are wound up onto a spool. Preferably, the spool-winding step comprises the step of winding the portions onto the spool so as to obtain the desired length of portions of ribbons which are free from residual torsion at a point downstream from the spool.